Pearce B, Morrow C, Murphy S. Characteristics of phorbol ester- and agonist-induced down-regulation of astrocyte receptors coupled to inositol phospholipid metabolism.
J Neurochem 1988;
50:936-44. [PMID:
2828550 DOI:
10.1111/j.1471-4159.1988.tb03002.x]
[Citation(s) in RCA: 58] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
We have examined some of the characteristics of phorbol ester- and agonist-induced down-regulation of astrocyte receptors coupled to phosphoinositide metabolism. Our results show that preincubation of [3H]inositol-labelled astrocyte cultures with phorbol 12-myristate 13-acetate (PMA) resulted in a time- (t 1/2, 1-2 min) and concentration-dependent (IC50, 1 nM) decrease in the accumulation of [3H]inositol phosphates (IP) evoked by muscarinic receptor stimulation. Much longer (30-40 min) preincubation periods with higher concentrations (IC50, 600 microM) were required to elicit the same effect with the receptor agonist carbachol. Following preincubation, agonist-stimulated [3H]IP accumulation recovered with time; in both cases pretreatment levels of inositol lipid metabolism were attained within 2 days. Both phorbol ester and agonist pretreatments were also effective in reversing the carbachol-evoked mobilisation of 45Ca2+ in these cells. However, their effects on phosphoinositide metabolism were found not to be additive. Although neither pretreatment affected the incorporation of [3H]inositol into phosphoinositides, both resulted in a loss of membrane muscarinic receptors as assessed by [3H]N-methylscopolamine binding. In washed membranes prepared from [3H]inositol-labelled cultures, the guanine nucleotide analogue, guanosine 5'-O-thiotriphosphate (GTP-gamma-S), caused a dose-dependent increase in [3H]IP formation. This response was enhanced when carbachol was also included in the incubation medium, although the agonist alone was without effect. Pretreatment with either PMA or carbachol had no effect on GTP-gamma-S-stimulated [3H]IP accumulation but did reduce the ability of carbachol to augment this response. Similar findings were obtained when membranes were exposed directly to PMA.(ABSTRACT TRUNCATED AT 250 WORDS)
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